Synthetic jellyfish a hybrid of rat hearts and plastic

An artificial version manges to match the real thing's performance.

With billions of years of tinkering behind it, biology has a bit of a head start on engineers, creating materials like geckoes' feet and spider silk that have remarkable properties. So some materials researchers have studied biology's output in order to try to match its performance, with a few notable successes. Now, a team from Caltech and Harvard have put together a device that's a mixture of biological material and plastic, and tweaked their design until it could swim like a jellyfish.

Swimming like a jellyfish isn't necessarily the most useful ability to have, but the simplicity of the jellyfish body plan makes for a somewhat easier engineering problem, and makes for straightforward comparisons between the performance of the test construct and the real thing. Plus, the research team has set its sights pretty high, focusing on building a hybrid device that's part plastic, part biological material.

The rough outlines of jellyfish swimming are pretty well understood, but the authors focused on swimming details of juveniles of the species Aurelia aurita. The first step in this swimming motion is a contraction of the organism's bell; in Aurelia, this is driven by a single layer of muscle cells that line the interior of the organism's surface. Once these relax, the slow spreading out of the bell is driven by the mechanical properties of the bell itself, which drive the organism back toward forming a flat disk. Incidentally, this sweeps new water immediately beneath the bell itself, which helps the organism feed.

Finally, the authors note that you can't simply consider the behavior of the organism alone; it's composed of viscous and solid material, and is moving through a viscous medium. The interactions at the boundary between the two play a key role in determining how everything works.

The authors decided to try to match the jellyfish as closely as possible by growing a single layer of muscle cells on a flexible surface made from plastic. Fortunately, cardiac cells, isolated from developing hearts, spontaneously form a single layer of tissue that will contract if grown in culture. So, the authors grew a layer of cells immediately on the plastic disk.

Normally, the contraction that drives the swimming stroke of a jellyfish is driven by sets of cells that act as pacemakers. To simplify matters, however, the authors used electrodes in the tank itself to run a voltage across the tank, which is enough to get the cardiac cells to contract.

It didn't work. The muscle cells simply couldn't generate enough force to bend the plastic, leading to the authors' first lesson in artificial jellyfish: "Successful medusoid designs must therefore match muscle stress generation with substrate compliance." So, they cut large slits out of the sides of their plastic, which allowed the remaining parts to contract without folding or cracking the disk. That turned out to work. For an hour after starting the experiment, the device would "swim" through the tank, driven by external electric currents. Based on measurements of its performance, the swimming stroke and "feeding" recovery were very similar to that of an actual jellyfish.

To get a better sense of how this works, you can have a look at the movies the authors have accompanying their paper by clicking on the DOI below (this one is especially informative). The authors term their successful creation a "medusoid."

The slits clearly allowed some fluid to escape during the contraction, but the viscosity of their test fluid (chosen after careful modeling) ensured that the slits weren't large enough to make the contraction ineffective. Making the slits wider, or dropping the fluid's viscosity too much would eliminate the construct's ability to swim.

Their concluding paragraphs make it clear that the authors intend to integrate more cell types into future versions of their medusoids. But they also suggest their general approach—having a good, functioning example, and understanding both the material and medium well—can be successfully applied to many other biomimetic efforts.